There is a lot of magic going on in a modern cockpit. Some of that magic isn't very modern at all, but it works great:
Everything here is from the references shown below, with a few comments in an alternate color.
Figure: Early Altimeters, from "Evolution of the Modern Altimeter".
Air is comprised mostly of nitrogen (78%), with some oxygen (21%), and a bunch of other gases. It can also have water vapor which displaces all that other stuff. The important takeaway here is that air is made of stuff and stuff weighs more than zero. All that air on top of you is pushing you down. More than that, it is pushing you all around. As you climb in altitude, even if just by foot when climbing up a mountain, there is less stuff on top of you, so less pressure.
You can measure just how much pressure is being exerted on you with a mercury barometer. You take a sealed glass tube with a vacuum at the top and a pool of mercury in a dish at the bottom. The mercury wants to slip out of the tube but the pressure of the air on the dish keeps it from doing so. If you measure the column of mercury you get a representation of how much pressure is being exerted in "inches of mercury."
More about this: Properties of the Atmosphere.
Hysteresis is simply "lag time."
This is what we had in the mighty T-37. It worked fine but you had to be careful with reading the correct pointer.
Figure Sensitive Altimeter Components, from Instrument Flying Handbook, Figure 3-3.
[Instrument Flying Handbook, pg. 3-3] The sensitive element in a sensitive altimeter is a stack of evacuated, corrugated bronze aneroid capsules like those shown in figure 3-3. The air pressure acting on these aneroids tries to compress them against their natural springiness, which tries to expand them. The result is that their thickness changes as the air pressure changes. Stacking several aneroids increases the dimension change as the pressure varies over the usable range of the instrument.
Have you ever wondered why a bag of potato chips is filled with air? Well chances are the "air" is nitrogen to keep them fresh and the bag appears to be mostly plump to keep the chips from being crushed when the bag is handled. The nitrogen is under a little pressure so the bag stays inflated. But I digress . . . Take that bag of chips with you on a flight and notice that as you climb the cabin altitude also climbs. As that happens the cabin pressure decreases. But even as the pressure around the bag decreases the pressure inside the bag stays the same. That means the bag appears to get even plumper. So what does this have to do with altimeters?
The aneroid inside the altimeter is composed of a stack of sealed chambers, just like that bag of chips. As the pressure around the aneroid decreases, the aneroid itself expands, moving the gears and levers that eventually move the needle on your altimeter. Flying a glass cockpit? Well it is the same principle, but the gears and levers are connected to electronic gizmos that feed the computers with the same information.
du Feu, A. N., "Evolution of the Modern Altimeter", Flight International, 26 Dec 1968, pg. 1066.
FAA-H-8083-15, Instrument Flying Handbook, U.S. Department of Transportation, Flight Standards Service, 2001.
Copyright 2019. Code 7700 LLC. All Rights Reserved.